Hematopoietic stem cell transplantations are increasingly employed to restore normal blood cell production after high-dose cytotoxic therapy in cancer patients. In addition, recent studies suggest that bone marrow-derived stem cells may have the potential to contribute to non-hematopoietic tissue regeneration after transplant, which potentially increases the clinical applications of these cells vastly. A significant problem in the field of stem cell therapy remains the inability to expand the number of multilineage long-term repopulating stem cells in vitro. In a large body of preliminary data we show that in the mouse long-term repopulating stem cells reside in the lineage-depleted, fibroblast growth factor receptor-positive cell fraction. Most importantly, fibroblast growth factor-1 (FGF1) has the unique ability to stimulate the generation of long-term repopulating stem cells in vitro in serum-free cultures. Thus, our data show for the first time that large-scale expansion of stem cells in vitro is feasible, and we provide a simple method to generate rapidly engrafting stem cells in vitro. However, we show that this expansion potential is highly mouse-strain specific, as C57BL/6 stem cells are easily expanded but DBA/2 stem cells are refractory to FGF1. Preliminary genetic linkage studies reveal that a locus on chromosome 11 is associated with the variation. It is the overall aim of the present proposal to investigate how incubation of bone marrow cells with FGF1 results in massive amplification of transplantable stem cells. To this end we defined 4 specific aims: 1. Identification of the target cell on which FGF1 exerts its activity, 2. Assessment of the clonal composition of hematopoiesis in recipients reconstituted with FGF1-expanded stem cells, 3. Determine whether the defect of DBA/2 stem cells is a cell-intrinsic trait, and 4. Investigate the genetic constrains that specify FGF1-induced stem cell expansion? In order to address these 4 issues we will use an extensive set of stem cell purification strategies to search for the FGF1-responsive cell. We will perform stem cell marking studies in which cells are retrovirally transduced prior to expansion and transplantation in lethally irradiated recipients. Using co-cultures of C57BL/6 and DBA/2 bone marrow cells we will evaluate whether DBA/2 stem cells are intrinsically impaired in their response to FGF1. Finally, using a genetical genomics approach we will search for an association between gene expression profiles and stem cell expansion potential in a set of 30 recombinant inbred BXD strains of mice.